This invention relates generally to diffractive imaging elements or lenses, including Fresnel zone plates, Fresnel phase plates, blazed Fresnel phase plates or other patterns which focus radiation primarily by diffraction, and more particularly to diffractive imaging elements having significantly reduced chromatic aberrations to produce a sharp focus and/or produce high quality images using much broader bandwidth radiation than is possible with conventional diffractive lenses.
Fresnel zone plates, Fresnel phase plates, and blazed Fresnel phase plates can be used to focus and/or image radiation. Blazed Fresnel phase plates are described in N. M. Ceglio and H. I. Smith, in "Proceedings VIII Int'l Conf. on X-Ray Optics and Microanalysis" (D. R. Beaman R. E. Ogilvie, and D. B. Wittry, Eds.), P. 255, Pendell, Midland, Mich., 1980. In addition, other diffractive optical elements (e.g., holograms or holographic optical components) can also be used to focus or image radiation. All these optical elements use primarily diffraction to achieve focus or image formation. Since diffractive power (e.g., focal length) is strongly wavelength dependent, all of these diffractive lens structures suffer from chromatic aberrations.
Fresnel diffractive structures (e.g., Fresnel zone plates, Fresnel phase plates, and blazed Fresnel phase plates) are divided into Fresnel zones where the radius of the nth zone is given by EQU r.sup.2.sub.n =nr.sup.2.sub.1 +n.sup.2 .lambda..sup.2 /4 (1)
where r.sub.1 is the radius of the central zone and .lambda. is the wavelength of the radiation to be focused. Such a Fresnel zone structure may be viewed as a diffractive lens having a focal length, EQU f=r.sub.1.sup.2 /.lambda. (2)
The focal length of the lens is wavelength dependent; indeed, the geometrical pattern itself (i.e., placement of the r.sub.n 's) is wavelength dependent. If a Fresnel structure designed to focus radiation at one wavelength, .lambda..sub.1, is used with radiation at a different wavelength, .lambda..sub.2, there will be a focal error or chromatic aberration (from equation (2)). The conventionally "acceptable" bandwidth for such a lens is generally taken as EQU .DELTA..lambda.=1/N (3)
wherein N=total number of zones. Under the conditions of equation (3), the performance of the lens is virtually diffraction limited and the focal spot size approaches the width of the outermost zone, EQU .DELTA.r=r.sub.N -r.sub.N-1 ( 4)
In practice, the acceptable bandwidth for illumination of the Fresnel structure will depend on the application, and will be determined by a trade-off between efficiency (i.e., accepting a broader bandwidth) and resolution loss (primarily due to chromatic aberrations).
Thus, diffractive lenses generally produce a sharp focus or a high resolution image only if illuminated with sufficiently narrowband (.DELTA..lambda..ltoreq..lambda./N) radiation. For these reasons, conventional diffractive imaging systems or lenses are generally viewed as narrowband imaging systems or imaging systems suffering from severe chromatic aberrations.
In many regions of the electromagnetic spectrum (and for other radiation such as neutrons, atoms, ions), refractive lenses are not practical so that diffractive lenses are all that are available to focus or image the radiation. This results either from severe absorption of the radiation in materials and/or because the refractive power of available materials is not sufficiently different from vacuum for those types of radiation. Under such circumstances, there is a great motivation for a scheme which would enable diffractive lenses to focus and/or image broadband radiation. Indeed, most sources of electromagnetic radiation in these spectral regions are broadband, for example, synchrotrons, plasmas, blackbody radiation, etc. Diffractive lenses also have properties which would make them very useful for application in parts of the electromagnetic spectrum where refractive optics already exist. For example, diffractive lenses have been easily implemented as bifocal and/or multifocal imaging and focusing elements. (Indeed, a bifocal or multifocal diffractive lens may simply be considered a hologram.) In addition, diffractive lenses can have high dioptric power and at the same time be very thin and easily deformable, making diffractive lenses attractive options for intra-ocular lenses and/or contact lenses. U.S. Pat. application Ser. No. 495,073 filed Mar. 19, 1990 describes microthin diffractive lenses for intraocular implants and corneal lenses. In these applications, it would be highly beneficial, and perhaps essential, that the diffractive lenses be able to focus and image broadband radiation and have significantly reduced chromatic aberrations.
There is, in addition, a great interest in x-ray optics in having relatively broadband imaging and focusing optics which can approach diffraction limited resolution. At soft x-ray wavelengths, Fresnel structures (zone plates and phase plates) have demonstrated diffraction limited resolution down to about 300A with narrowband (.DELTA..lambda.&lt;.lambda./N) illumination. The best performance for broadband imaging has been achieved using grazing incidence reflection optics, and image resolutions of order.gtorsim.1 .mu.m have been demonstrated. There are applications in x-ray microscopy, materials analysis, and x-ray matter interaction studies which could benefit from an ability to focus and/or image relatively broadband radiation with diffraction limited or near diffraction limited performance.
With such a strong motivation for broadband diffractive optics, there have been various attempts to design diffractive lens doublets or triplets to correct for and/or reduce the chromatic aberrations in diffractive optics. These approaches to chromatic aberration correction are less than satisfactory for at least two reasons: (1) They generally involve two or more diffractive elements separated by a finite distance. As such, they are really a "lens system" or an "optical system", not a simple, compact broadband lens. For many applications (e.g., contact lens or intra-ocular lens implants), the "system" approach is not practical. (2) In addition, diffractive optical elements typically operate at limited efficiency. For example, an ideal Fresnel zone plate diffracts only 10% of the incident (narrowband) radiation into its first order focus, and an ideal Fresnel phase plate (in the absence of radiation absorption) diffracts 40% of the incident narrowband radiation into its first order focus. [However, if the Fresnel phase structure is appropriately blazed (i.e., a blazed Fresnel phase plate) it can, in principle, direct 100% of the incident radiation into its focal spot.] Thus, an optical system for chromatic aberration correction that puts M such structures (each having efficiency .gamma.) in series suffers in overall radiation transport efficiency by a factor of (.gamma.).sup.M. For example, a triplet (M=3) of zone plate structures (.gamma.=0.1) would have an overall efficiency of 0.001.